Means for electronically changing the pitch of a musical instrument or the human voice, commonly referred to as a "speed up loop", "pitch changer", "slow down loop" or "Harmonizer (TM)" was first developed over ten years ago. These devices are used extensively in recording studios to correct musical pitch and in variable speech control devices to correct picth while varying a tape recorder's playback speed.
These known devices employ a means for quantizing the speech or music audio, storing the quantized analong signal in a memory by means of a write vector, and reading the memory space with a read vector, converting the quantized read signal to analog form and playing out the read analog signal by means of an audio amplifier and loud speaker. In these known devices, the write and read vectors rotate through the memory space in the same direction with variable angular velocities. If both vectors have the same angular velocities, no-pitch change occurs. However, delays can be effected by varying the angular separation between the two vectors, thereby accomplishing phase or time delays in the signal output relative to the signal input. If the write vector angular position moves more than the read vector in the same time period, then the pitch in the output signal decreases. Conversely, if the read vector rotates faster than the write vector, then the pitch in the output signal increases. For either case, the pitch change is proportional to the ratio of the two angular velocities.
This prior art exhibits limitations wherrein the write and read vectors intercept each other in the memory space. A discontinuity commonly referred to as a "glitch" occurs in the read signal at the intercept point. This glitch produces unwanted noise frequencies in the output at the glitch rate and its harmonics. Digital signal processing algorithms and special hardware filtering methods have been developed to de-emphasize the noise glitch.
This prior art exhibits further limitations wherein the output signal pitch is a fixed musical interval away from the input reference pitch. Musical intervals of 5ths, 3rds, octaves, etc. may be selectable, however, the output remains fixed for the selected interval. Multiple devices are required to create a trio or quartet from a single voice input. Associated switching is required to vary the chord structure eg. dominant 7th, tonic, augmented, diminished, etc.
This prior art exhibits even further limitations when the output pitch is lowered with respect to the input pitch. In this instance, information is lost since the write vector overwrites information that is never read by the read vector because the write vector is traveling faster then the read vector through the memory space.